^But that doesn't explain how a timeline can on the one hand be "close" in having the same individuals being born and ending up working in the same ship or station with the same design, yet on the other hand be extremely different in its history and culture. There's more going on there than arbitrary probabilistic proximity. By your model, the timelines that Trek characters would cross into wouldn't be things like the Mirror Universe where the history is different but the same people and ships exist. They'd be things like the early "Parallels" alternates where the people and events are largely the same aside from minor details, or -- as you get further away -- ones where the overall history is generally the same but some different people are born or end up in different jobs. Something where the large-scale history is so enormously different that you have an empire instead of a Federation, but the same individuals are still being born and meeting each other and going to work in the same place -- that's a paradox. The ease of access can't be explained by some simple model of "closeness," because it's simultaneously very close in some ways and very distant in others.